1. Field of the Invention
The invention relates to the use of reflected signals from satellites to gather information. In particular, the invention relates to the utilization of reflected satellite signals to determine information about traffic or about a geographic area.
2. Description of the Related Technology
There are various types of satellites that transmit signals to the Earth. Some examples of satellites that transmit signals to the Earth are high orbit satellites (e.g. a DirecTV® satellite), middle orbit satellites (e.g. a Global Positioning System (GPS) satellite), and low orbit satellites (e.g. low Earth orbit (LEOS) satellite altimetry systems).
High orbit satellites, such as DirecTV® are geo-stationary satellites that are positioned at an altitude of over 35,000 km. High orbit satellites transmit a constant output of signals.
Low orbit satellites, such as LEOS satellite altimetry systems have an altitude of below 1,500 km. Low orbit satellites are non geo-stationary. Low orbit satellites transmit a rapidly changing output of signals.
A middle orbit type of satellite has an altitude of around 1,500 km to 35,000 km. They are pseudo geo-stationary and can provide a stable output of signals. GPS is a satellite-based global navigation system that was made possible by the U.S. Department of Defense. The 24 GPS satellites or space vehicles (“SV's”) are traveling at a constant speed of roughly 7,000 miles an hour, in a nearly circular orbit at a height of about 12,500 miles (or 20,200 km) above the Earth's surface, and make two complete orbits every 24 hours. The satellites, which transmit GPS signals down to the Earth's surface are positioned in such a way that signals from at least 4 satellites are detectable at any place with a fairly good view of the sky at any moment of time. The GPS receivers detect and process the GPS signals and determine position (latitude, longitude, and altitude), velocity and precise time from the information bearing signals. With an appropriately programmed GPS receiver, the location of the GPS receiver can be determined by triangulation.
To get a better accuracy or precision, civilians may use a technology called Differential GPS (“DGPS”). DGPS works by placing a high performance GPS receiver (also known as reference station) at a known location, where the errors in the satellite signals are determined. The reference station then sends correction signals to GPS receivers. The GPS receivers improve accuracy by eliminating most of the satellite signal errors through use of the information sent by the reference station.
GPS technology has proven invaluable for both military and civilian purposes such as mapping, surveying, tracking, and navigation.
Since GPS was designed for use by an unlimited number of users, military or civilians, at the same time, the structure of the signals, as well the structure of the receiver is quite complex. Each of the GPS satellites continuously and simultaneously transmits two microwave signals in the UHF band, denoted by L1 and L2 respectively. The mathematical models for L1 and L2 signals (SL1(t) and SL2(t) respectively) are as follows:SL1(t)=√{square root over (2)}√{square root over (PI)}d(t)c(t)cos(2πf1t+θ1)+√{square root over (2)}√{square root over (PQ)}d(t)p(t)sin(2πf1t+θ1)SL2(t)=√{square root over (2)}√{square root over (PQ)}d(t)p(t)sin(2πf2t+θ2)where PI is the in-phase carrier power, PQ is the quadrature-phase power, d(t) is the 50-bps navigation data stream, c(t) is the pseudo-random coarse/acquisition code (or C/A-code), p(t) is the pseudo-random protected code (or P-code), θ1 and θ2 are arbitrary phase angles, f1=1575.42 MHz and f2=1227.60 MHz. The carrier of L1 is composed of an in-phase and a quadrature component, whereas L2 contains only the quadrature part. The in-phase component is bi-phase modulated by the data stream and the C/A-code. The data stream contains information such as satellite almanac data (used to determine which satellites are visible at a given location), satellite ephemeris data (used to determine the position of the satellites), and signal timing data. The C/A-code is a 1023-chip pseudorandom sequence, has a period of 1 ms (thus, the chipping rate is 1.023 MHz), and is unique for each satellite. Its purpose is to spread the spectrum of the data message and to prevent co-channel interference from other satellites. Different from the in-phase part, the quadrature component is modulated by P-code. The P-code is also pseudorandom, but has a much longer period (1-week) and a higher chipping rate (10.23-MHz). The use of P-code offers better jamming protection, more resistance to errors, and thus better accuracy than just with the C/A-code. However, civilians have no access to the P-code since it is encrypted. As a result, civilian applications use L1 only. The power a user receives on the ground for L1 is expected to be at least −134.1 dBm (or 10−13.41 mW, Table 1) prior to adding any gains of the receiver.
TABLE 1Link Power Budget of GPS SignalsParametersL1L2Sending End (Satellite)Transmitted Power10.72W or6.61W or40.3dBm38.2dBmAntenna Gain13.5dB11.5dBEIRP53.8dBm49.7dBmLossPolarization Mismatch Loss3.4dB4.4dBAtmospheric Loss2.0dB2.0dBFree-Space Propagation Loss182.5dB180.3dBMinimum User Received Power−134.1dBm−137.0dBmon Ground without Antenna Gain
Note that in Table 1: 1) EIRP (effective isotropic radiated power) is obtained by adding transmitted power in dBm and antenna gain in dB; 2) the free space loss is calculated and shown in Table 2; and 3) the minimum user received power is determined by subtracting all losses from EIRP.
TABLE 2Free Space Loss CalculationFree-Space Propagation Loss Calculation> restart; Loss:=(4*Pi*d/lambda){circumflex over ( )}2;   Loss  :=      16    ⁢                  ⁢                            π          2                ⁢                  d          2                            λ        2             > lambda:=c/f: Loss;   16  ⁢          ⁢                    π        2            ⁢              d        2            ⁢              f        2                    c      2       Using the actual parameters: speed of light c = 30000 km/s, frequency ofL1 carrier f = 1575.42 MHz, distance from the satellite to the groundd = 20200 km,> c:=3E8: f:=1575.42E6: d:=20200E3: Loss:=evalf(Loss);Loss := .1776939602 1019> LossDB:=10*log10(Loss);LossDB := 182.4967267
The structure of a GPS receiver is shown in FIG. 1. The GPS signal is processed in a number of stages: once at the RF (Radio Frequency) stage and twice at the IF (Intermediate Frequency) stages. In those stages, the signal is amplified and/or down-converted a few times before it is digitized. The down-conversions are done for the purposes of amplification, noise filtering, and ease of digitization. The down-conversion is essentially the process of mixing the incoming and the local oscillator (LO) signals, which results in a signal at a lower frequency or at IF (IF is the difference between the incoming and LO frequencies). The number of stages of IF signal-processing can be one or more. But, for a relatively low cost receiver, multiple IF processing is usually preferred. The digitized signal is then further processed to give navigation information such as position, velocity, and time.
Various methods for using GPS signals to assist in determining traffic information have been previously suggested. Some of the previous methods are discussed below.
U.S. Pat. No. 6,650,948 B1 to Atkinson discloses a method for monitoring traffic flow. GPS signals are received by devices placed aboard vehicles. This information received from the GPS signals is then used to create probability vectors to predict traffic patterns. This implementation requires each vehicle to have a GPS receiver on board.
U.S. Pat. No. 6,615,130 B2 to Myr discloses a system for using GPS signals for determining traffic related information. A central traffic unit correlates various incoming information from vehicles on a road. Vehicles receive GPS signals and these signals are then transmitted to a central traffic unit. The central traffic unit then uses this information to calculate the density and speed of traffic on a road. This system requires that the monitored vehicles have GPS receivers and transmitters.
U.S. Pat. No. 6,334,086 B1 to Park discloses a system for using GPS signals to determine traffic patterns. Probe vehicles are used to receive GPS signals, and stationary devices are employed at various locations to receive signals from a probe vehicle in order to identify a road on which a car is traveling. This system requires the monitored vehicles to have receivers and transmitters and further requires placement of a number of stationary devices to identify the particular roads on which the vehicles are traveling.
While the systems disclosed above utilize GPS signals to determine information about traffic, each of these systems requires the active participation of vehicles at least to receive GPS signals, and, in some cases, to transmit signals as well. Placement of GPS receivers and/or transmitters in individual vehicles to gather traffic data is a costly way of determining traffic information. In order for these devices to be effective, enough monitored vehicles must be present on any given road in order to give a statistically reliable result.
U.S. Patent Publication No. 20030171872 by Balasubramanian et al. discloses the use of GPS to determine the condition of a road. The device uses reflected GPS signals to determine road surface conditions. The signals are used to determine whether the scanned surface is wet, dry, or is covered by ice, snow, sand or the like. The receiver of the signals may be mounted on a vehicle or provided at or near the location of the road to be monitored. When provided proximate to the road, a signal is sent to a passing vehicle that has an apparatus on board to evaluate the signal. This will inform the vehicle occupant as to the condition of the road.
While the Balasubramanian publication discloses detecting a condition of a road using reflected signals, using the signals to determine information about the traffic on that road is not contemplated.
Therefore, there remains a need to provide an improved method of determining information about traffic without requiring each monitored vehicle to have one or more devices installed in the vehicle in order to participate in the traffic monitoring process.
There also remains a need to provide a more efficient method of determining information about a particular geographic area.